Just blind shot here, i dont know if screen orientation isnt fixed and i dont have Android nor iPhone to test it out, but i think you can hack it with simple script:
extends Node2D
signal orientation_changed(portrait)
var last_size
func _ready():
set_process(true)
last_size = get_viewport().get_rect().size
func _process(delta):
var size = get_viewport().get_rect().size
if size.x != last_size.x and size.y != last_size.y:
emit_signal("orientation_changed", size.x > size.y) # maybe its < ? :)
last_size = size